1. Field
This document relates to an organic light emitting diode display and a method for compensating the chromaticity coordinates thereof.
2. Related Art
An active matrix type organic light emitting diode display (AMOLED) is attracting a lot of attention as a next generation display because of advantages of fast response speed, high light emission efficiency, high luminance, and wide viewing angle. The organic light emitting diode display displays an image by controlling a current, flowing in an organic light emitting diode (hereinafter, OLED) by using a thin film transistor (hereinafter, referred to as “TFT”).
A typical organic light emitting diode display has a plurality of pixels, each comprising an R (red) sub-pixel, a G (green) sub-pixel, and a B (blue) sub-pixel for full-color displays. An R emission layer EML for generating red light is formed in the OLED of the R sub-pixel, a G emission layer for generating green light is formed in the OLED of the G sub-pixel, and a B emission layer for generating blue light is formed in the OLED of the B sub-pixel. An emission layer is deposited separately for each sub-pixel by a fine metal mask (FMM) method using a metal mask, etc. However, the larger the size of the substrate, the more the mask is bent. Thus, the conventional deposition method using a metal mask decreases yield because it makes it difficult to precisely pattern an emission layer. As a result, it is hard to apply this method to large area and high precision displays.
As such, in recent years, the technology of implementing a color display device using a white OLED is emerging which does not require the use of a metal mask during the formation of an emission layer in an organic light emitting diode display. The white OLED has a structure in which an R emission layer, a G emission layer, a B emission layer, etc. are optionally laminated between a cathode and an anode. The white OLED is formed for each sub-pixel. This organic light emitting diode display has a plurality of pixels, each comprising an R sub-pixel, a G sub-pixel, a B sub-pixel, and W (white) sub-pixel for color displays. The R sub-pixel comprises an R color filter for transmitting red light among white light incident from the white OLED, the G sub-pixel comprises a G color filter for transmitting green light among white light incident from the white OLED, and a B color filter for transmitting blue light among white light incident from the white OLED. The W sub-pixel has no color filter, and transmits entire white light incident from the white OLED to compensate for a decrease in image luminance caused by the color filters.
Such an organic light emitting diode display generates W data based on R data, G data, and B data input from the outside, and modulates the R data, the G data, and the B data using the generated W data. The W data, the modulated R data, the modulated G data, and the modulated B data are respectively displayed in the W, R, G, and B sub-pixels.
The aforementioned conventional art was proposed under the assumption that the chromaticity coordinates of the white OLED are uniform. However, in reality, the white OLED displays a white color by a combination of emission layers of several colors. Thus, color changes vary according to the driving voltage of the material used, and this disturbs the color balance of white. This leads to a shift in white chromaticity coordinates for each gray level when emitting only W sub-pixels in the conventional art.
For example, in a panel where target values of the chromaticity coordinates (x, y) are set to (0.290, 0.300), the chromaticity coordinates (x, y) of a target luminance L for each gray level are different from the target values (0.290, 0.300) given in FIG. 1 due to the device characteristics of the white OLED. In particular, the degree of a shift becomes larger toward low gray levels as shown in FIG. 2, thus causing a yellowish phenomenon in low gray levels. There is a demand for a method for preventing the distribution of white chromaticity coordinates for each gray level and converging them to predetermined target values, as shown in FIG. 3, in an organic light emitting diode display using a white OLED.